1. Field of the Invention
This invention relates to a polyion complex separation membrane with a double structure, which is applicable in the separation process of a water-soluble mixture having ionic molecules or particles by means of reverse osmotic pressure for the purposes of recovering expensive ionic organic materials such as anionic emulsifiers from the waste water. More particularly, this invention relates to said membrane having anionic polymers, as substrate, which is immersed into a cationic polymer solution containing a multivalent ion cross-linking agent, by which a cross-linkage within the internal of the separation membrane is achieved, thereby forming an ion complex between ionic polymers of opposite ion at the surface of the separation membrane to yield a stable separation membrane of a double structure.
2. Description of the Prior Art
In general, a mixture consisting of water and ionic organic compounds may be separated using the method of reverse osmotic pressure or low-pressure reverse osmotic pressure, in addition to the method of permeation evaporation.
Anionic emulsifier, one of the ionic organic materials, is utilized for the purposes of preparing water-soluble emulsions of polytetrafluoroethylene during emulsions polymerization. The water-soluble emulsion, so obtained from the said emulsions polymerization, is then concentrated. After filtration and separation of the solution with dissolved emulsifier, all of the filtrate solution undergoes waste-water treatment.
The methods of recovering emulsifiers in the filtrate solution include water evaporation (U.S. Pat. No. 3,316,201), vapor distillation, ion exchange by ion-exchange resin (U.S. Pat. No. 3,668,167), freezing, and solvent extraction. The simplest method is based on the water evaporation, as disclosed in U.S. Pat. No. 3,316,201, wherein the filtrate is concentrated to about 10% thereof, and after rejecting some of the impurities by separation of the layers with sulfuric acid, the filtrate is again distilled. However, water evaporation method has some disadvantages, for example, requires a lot of energy consumption and evaporation time is increased. The vapor distillation method involves expensive purification costs due to the usage of steam. The ion-exchange method requires a proper selection of ion exchange resins with complicated processes including adsorption and desorption.
By contrast, the use of membrane separation process by reverse osmotic pressure may save energy and other costs associated with the recovery process of emulsifier. The above separation method may be applicable to metals/inorganic ion solution, organic ion solution, and organic material solution.
As far as the substrate for the separation membrane are concerned, the strong hydrophilic polymers have been mainly used for the selective permeation of water. As for the separation of the solution containing ions, the ionic polymers or ion exchange resins have been mainly used as the substrate for the separation membrane so as to facilitate the separation between water and the solute by using the electrical attraction, e.g., electrical potential, between the ionic separation membrane and ionic molecules dissolved in the solution.
Further, in order to have water absorbed selectively in the separation membrane and then to have water so absorbed to permeate the separation membrane, the solubility of the separation membrane with respect to water must be excellent. More specifically, by raising the selectivity and permeability of the separation membrane to water, the separation performance of the separation membrane can be improved. The commonly used separation membrane substrates include polyvinyl alcohol (J. Appl. Polym. Sci., 50 (1993) 1013.about.1034), polyacrylic acid (J. Appl. Polym. Sci., 41 (1990) 2133.about.2145), cellulose-based polymer (J. Membr. Sci., 106 (1995) 245.about.257), and amide-based polymers (J. Membr. Sci., 114 (1995) 39.about.50). These polar polymers have good hydrophile with strong hydrogen bonds.
Meanwhile, as for the separation of the ionic solutes contained in a solution, by endowing the separation membrane with the same property as such ionic solutes, the molecules of the solute may be effectively excluded due to the electrostatic repulsion thereof. However, if the polarity of the polymer is increased, the hydrophile is also increased, resulting in the reduction of the mechanical property and stability of the separation membrane due to swelling induced by water. Such drawbacks may be prevented by the method of a chemical modification or polymer mixture designed either to introduce a cross-linking structure to the separation membrane or a hydrophobic group to the backbone or side chain of the polymer. In such case, the stability of the separation membrane is enhanced at the expense of the deterioration of the separation performance of the separation membrane due to the reduction in the hydrophile.
To minimize the reduction in the hydrophile and provide better stability to the separation membrane, the method has been devised to form ion cross-links within the internal of the polymer by using multivalent ion-based inorganic ions or metals ions as cross-linking agent to the polymers, such as polyvinyl alcohol, polyacrylic acid, and polysaccharide. However, such method needs to be further improved in that when such cross-linked separation membrane is exposed to the flow of supply solution for a prolonged period of time, the cross-linkage ions are washed way by the solution, thus deteriorating the stability of the separation membrane with lapse of time, or the pH of the solution is adversely affected thereby.